The present invention relates to a fly-by-wire flight control system, and more particularly to an airspeed hold system.
Fly By Wire (FBW) flight control systems are emerging as state of the art in control of rotary-wing aircraft. FBW permits a wide range of improvements that contribute to pilot workload reductions. Although significant benefits are realized with FBW systems, advanced system hardware may alter the pilot-vehicle interface in such a way that civil certification requirements may not be met. Special provisions must then be made in the control system to meet these requirements, which may compromise performance benefits of FBW.
The static position of the longitudinal cyclic flight controls which control aircraft pitch altitude, change as a function of aircraft flight conditions. The pilot typically retrims the forces felt on the stick each time the flight conditions (speed of forward travel, climb/decent angle) are changed. Such retrimming minimizes or cancels out the static control forces to increase pilot comfort and flying accuracy. However, the need to constantly retrim the forces on the cyclic stick may itself increase pilot workload. To reduce this workload, FBW systems are introducing advanced control laws and unique trim control devices which allow the pilot to center the control stick when the helicopter is not being maneuvered. These FBW changes mask the relationship between airspeed, rotor disk position, and cyclic stick position, and pose particular complications to FAA certification.
In a rotary wing aircraft, forward longitudinal cyclic pitch generally increases with airspeed. That is, the equilibrium longitudinal position of the cyclic stick in a conventional control system generally represents the speed of forward travel: a forward position corresponds to a higher steady forward speed, a rearward position to a lower steady forward speed. This also imposes a consistency of action between short-term commands and long-term commands (equilibrium) for variation in speed. In the short term, a demand to increase speed requires a pitch-down command (tilting the rotor disk forward) and a shifting of the cyclic stick forward. Furthermore, some rotary-wing aircraft may have neutral or negative longitudinal static stability in a certain speed range. This speed range generally results in an increase in pilot workload as the position of the cyclic stick in equilibrium moves back as the speed increases for a constant collective pitch. To change from one steady speed to another, higher one, without touching the collective pitch lever, the pilot has first of all to push the stick forward (pitch-down command) then bring it back into a static position further back than the initial position in order to stabilize the speed. Also, following an external disturbance and in the absence of a corrective action from the pilot, the aircraft may tend to drift away from its steady speed without tending to return (velocity disturbance), at least until the aircraft returns to a region of positive static stability, if one exists.
In order to correct such instability, various conventional trim compensation systems are utilized. One conventional trim control system mechanically offset the position of the cyclic stick. However, this approach only provides the pilot with apparent static stability without improving the velocity disturbance rejection capabilities of the aircraft. Another conventional trim control system for military-type rotary-wing aircraft utilize complex neural networks which, although effectively predicting aircraft flight regimes, will not provide a seamless transition between Rate Command/Attitude Hold response and an Airspeed Hold function. Such systems are exceedingly complex, specific to certain aircraft and are not practical for FAA certification.
Accordingly, it is desirable to provide a FBW static longitudinal stability system which provides an unobtrusive airspeed hold function that reacts to pilot control inputs and the measured states of the aircraft to engage and disengage smoothly without any explicit mode selection by the pilot when the aircraft is in a trimmed, non-accelerating state.